Recently, development of next generation clean energy sources has become a crucial issue to overcome severe environmental contamination and depletion of fossil fuels. In particular, solar cells directly converting sunlight into electric energy have various merits such as avoidance of contamination, infinite resource and semi-permanent lifespan, and are thus anticipated as an energy source capable of solving the problem of energy depletion.
Solar cells are classified into five types depending on materials for a photo-absorption layer, and a silicon solar cell is most widely used in the art. Recently, however, a rapid increase in raw material costs due to undersupply of silicon has led to increasing interest in thin film solar cells. Thin film solar cells are manufactured to a low thickness, thereby providing merits such as low consumption of material, light weight, wide application ranges, and the like. As for materials of such a thin film solar cell, studies have been actively conducted on amorphous silicon, CdTe, CIS, CIGS, and the like.
A CIS or CIGS thin film is aI-III-VI compound semiconductor and has a higher conversion efficiency of about 19.9% than any other thin film solar cells. In particular, since the CIS or CIGS thin film can be formed to a thickness of 10 micrometers or less and is stable after long term use, the thin film is anticipated as an inexpensive, highly efficient solar cell capable of replacing silicon.
Particularly, the CIS thin film is a direct transition type semiconductor and may be formed into a thin film. Further, the CIS thin film has a band-gap energy of 1.04 eV suited to photo conversion and exhibits a higher coefficient of photo absorption than any other solar cell materials known in the art.
The CIGS thin film is developed to replace some of indium In by gallium Ga or replace selenium Se by sulfur S to improve a low open circuit voltage of the CIS thin film.
Generally, a CIGS solar cell is formed using a thin film having a thickness of several micrometers by vacuum deposition, or by precursor deposition in a non-vacuum state and heat treatment of the precursor-deposited thin film. Vacuum deposition is advantageous in that it provides a highly efficient absorption layer. However, vacuum deposition disadvantageously provides low uniformity and requires expensive equipment in forming a large area absorption layer, and causes material loss of about 20% to 50%, which leads to high manufacturing costs. On the other hand, formation of the solar cell through a non-vacuum precursor deposition and heat treatment at high temperature may advantageously reduce manufacturing costs while providing a uniform large area. However, this process is disadvantageous due to low efficiency of the absorption layer.
Since the CIGS thin film formed by depositing a precursor in a non-vacuum state has lots of pores and exhibits non-dense characteristics, selenization heat treatment is performed. Here, since conventional selenization heat treatment is performed using toxic hydrogen selenide (H2Se) gas, huge installation costs are required to provide a safety system to guarantee safety and the heat treatment is performed for a long duration, thereby causing an increase in the unit cost of the CIGS thin film.
Further, since the CIGS thin film has very high melting point of 1000° C. or more, it is difficult even for CIGS particles having a size of dozens of nanometers to allow particle growth and densification through post-heat treatment.
The inventors of the present invention carried out diligent studies to obtain a method of manufacturing a high density CIS thin film, CIGS thin film or CZTS thin film through non-vacuum coating. As a result, the inventors found that a high density CIS thin film can be formed when heat treating the CIS thin film, CIGS thin film, CZTS thin film, with cavities of the film filled with filling elements such as copper, indium, gallium, zinc, tin, and the like. Accordingly, it was also found that, when such a high density CIS thin film was applied to a photo-absorption layer of a thin film solar cell, it is possible to improve efficiency of the thin film solar cell while reducing manufacturing costs thereof.